One of the major challenges in image projection is the representation of three-dimensional images by means of stereoscopic methods. In this case, the left and right eyes are intended to perceive different images. Overall, such a projection device is intended to be cost-effective to realize and to be distinguished by high robustness. Particularly in the case of so-called pico-projectors, therefore, 3D projection can be realized only with difficulty. Pico-projectors are required in particular for incorporation into cellular phones and other mobile telecommunication devices.
Known 3D projection methods use special spectacles for this purpose. In this regard, so-called shutter spectacles are used, for example, wherein the images of the two channels, that is to say of the channel for the right eye and of the channel for the left eye, are represented alternately on the projection surface and in addition, the left eye and respectively the right eye are alternately shaded in a manner synchronized with the image projection. However, flicker phenomena can very easily occur here if the image frequency is too low. Moreover, such spectacles require a dedicated power supply.
In a second known method, different polarization filters are applied on the two spectacle lenses in order to filter the respective light channel. In the linear polarization method, the two polarization filters used have a crossed polarization axis, that is to say that the two directions of polarization are perpendicular to one another; in the circular polarization method, the polarizations of the two spectacle lenses are in opposite directions. In this regard, the image information for each eye can be defined solely by way of the polarization. As a result, in principle, two images can be represented simultaneously and the probability of flicker effects is equal to zero. In the related art, such a method is used in cinema projection, for example. Two projectors each having at least one discharge lamp are usually used in this case. A corresponding polarization filter is placed in front of each projector. Such a method takes up a great deal of space and therefore cannot be used in small projectors, for example for cellular phones and other mobile communication devices.
With regard to the related art, reference is furthermore made to the subsequently published German application in the name of the same applicant as the present application, application no 10 2011 087 184.5, which proposes a projection device illustrated in FIG. 1. Such a projection device 10 includes six laser diodes 12, for example. In this case, laser diodes 12 which emit a radiation in the same wavelength range are respectively arranged opposite one another. In the present case, the laser diodes 12B1 and 12B2 emit radiation in the blue wavelength range, the laser diodes 12G1 and 12G2 emit radiation in the green wavelength range, and the laser diodes 12R1 and 12R2 emit radiation in the red wavelength range. A lens 14B1 to 14R2 is assigned to each laser diode 12B1 to 12R2. A prism 16 is arranged between the respective opposite laser diodes 12. Said prism in the present case includes three polarization-dependent and wavelength-dependent mirror devices 18, wherein a first 18B is arranged between the laser diodes 12B1 and 12B2, a second 18G is arranged between the laser diodes 12G1 and 12G2, and a third 18R is arranged between the laser diodes 12R1 and 12R2. The respective wavelength-dependent, polarization-dependent mirror device forms an angle of α=45° with the connecting line between the respective laser diodes.
In the example illustrated, the radiation emitted by the respective laser diodes 12 is s-polarized. In the example illustrated, the wavelength-dependent, polarization-dependent mirror devices 18 are designed to reflect s-polarized radiation and to allow p-polarized radiation to pass through. In the respective polarization-dependent mirror device 18, this effect occurs only in a wavelength range provided for this. Radiation of other wavelengths can pass through the wavelength-dependent, polarization-dependent mirror device 18 substantially without reflection, i.e. apart from unavoidable minimal losses.
After reflection at the respective mirror device 18, radiation emitted by the laser diodes having the index “2” is accordingly reflected in the direction of the output A of the projection device. By contrast, radiation from the laser diodes having the index “1” is reflected in a direction opposite thereto by 180°, to be precise in the direction of a λ/4 retardation plate 20. The respective radiation penetrates through the λ/4 retardation plate 20 and impinges on a mirror device 22 designed to reflect all radiation in the three wavelength ranges indicated. After reflection at the mirror device 22, the radiation passes through the λ/4 retardation plate 20 again. On account of passing through the λ/4 retardation plate 20 twice, the direction of polarization has changed from “s” to “p”. The now p-polarized radiation enters into the prism 16 again, wherein the radiation can now pass through the wavelength-dependent, polarization-dependent mirror devices 18 in an unimpeded manner, on account of its p-polarization, in the direction of the output A.
Accordingly, at the output A, a laser beam is available in which s- and p-polarized laser beams are superimposed relative to the respective wavelength range. At the output A, a so-called micromirror arrangement, for example an MEMS arrangement (MEMS=MicroElectroMechanical System) can be provided in order to project the cumulative radiation onto a projection surface. A user who views the projection surface and who wears cross-polarized filter spectacles can perceive a colored 3D representation.
Even though three pairs of laser diodes opposite one another were used in the example illustrated, the concept can also be realized with only one pair of laser diodes opposite one another or even a larger number of laser diodes opposite one another. Equally, the radiation emitted by the laser diodes can be p-polarized, such that its polarization axis is rotated by 90° after passing through the λ/4 retardation plate twice. The previously p-polarized radiation is now s-polarized. The same result as with the example illustrated in FIG. 1 can thus be achieved. Furthermore, other ways of modifying the polarization are possible instead of the λ/4 retardation plate, for example 3λ/4 retardation plates and the like.
In accordance with another exemplary realization, the two laser diodes arranged opposite one another can be laterally offset, that is to say not arranged on a common axis. Mutual influencing of the laser diodes can largely be precluded as a result. The λ/4 retardation plate can be applied directly on the prism. The mirror device can be embodied as a prism having a reflectively coated outer surface, wherein the prism can be applied directly on the λ/4 retardation plate. By virtue of a suitable choice of the angles of the prism, it is possible, in this respect see FIG. 2 of the application in the name of the present applicant filed under the internal file reference 201117170, for the respective beams to be superimposed and provided in this superimposed form at the output A. It goes without saying that such an example can also be extended by further pairs of laser devices.
Generally, that is to say independently of the realization chosen, the following can be noted: the respective laser devices can emit their radiation simultaneously. Alternatively or supplementarily thereto, the laser devices can also emit their radiation in a sequentially time-shifted manner for example with a predefinable clock rate.
The retardation plate can also be attached to the prism without any spacing, that is to say directly. Instead of a plate-type structure, it is also possible to use a retardation film, for example a λ/4 retardation film having a thickness of approximately 50 μm.
The mirror device can be configured as a highly reflective mirror and can be arranged at a distance from the retardation plate, or can be applied or fitted for example as a highly reflective coating directly on that side of the retardation plate which faces away from the laser devices.
The mirror device can also be configured as a prism.
The laser devices are not restricted to the use of laser diodes. Rather, they can encompass all types of polarized laser light sources, that is to say in particular including gas lasers, solid-state lasers or fiber lasers.
The inventors of the present application have ascertained that unstable operation of the laser diodes involved can occur in realizations of the projection devices mentioned.